This invention relates to a voltage (V), current (I), and power (P) meter for solar arrays of photovoltaic cells.
Photovoltaic cells designed to convert solar energy into electrical energy have been developed sufficiently for commercial use in only special applications where other commercial power sources are not readily available, such as on oil platforms and at microwave relay stations on top of mountain peaks. This is due primarily to the present high cost of the solar cells. Consequently, the only places where solar arrays are cost effective are places where an unattended source of power is required, namely the most difficult places in the world to reach.
Evaluation of array performance and diagnosis of faulty solar cell modules requires compact portable instrumentation for measuring array output voltage, current and power (VIP) characteristics. Instrumentation is not available that is sufficiently portable for a technician to check the VIP characteristics of an array under remote field operating conditions.
A solar cell is essentially a diode, and if a large number of such cells are connected together in an array, the array characteristics look very much like the diode characteristics. The level of short circuit current remains substantially constant as output voltage across a load increases toward an open circuit until a region is reached known as the "knee" of the characteristic curve. There current begins to drop, and the rate of drop increases very rapidly at levels very near the maximum output voltage of the array.
The I-V characteristic of a solar array will degrade in different ways due to different defects which develop in the array through normal use. For example, the cells of the array are normally encapsulated in highly transparent material, but the transparency of the material degrades with age due to yellowing of the material itself and the accumulation of dust on its surface. This type of degradation decreases the short circuit current of the array, i.e., decreases the current amplitude of the I-V characteristic. This is because the short circuit current characteristic of the array is essentially of precise linear dependence on solar intensity. This is well recognized; in fact, solar cells are used individually or in small arrays in commercial light meters.
Another type of degradation of an array manifests itself as an increased series resistance in the array which causes the current to decrease earlier and faster as the load voltage across the array is increased. This is commonly referred to as "softening of the knee" which is readily apparent in a plot of the I-V characteristic by the collapse of the normal well-defined knee of the characteristic inwardly towards the origin of the I-V graph. The extent to which the knee collapses due to this type of degradation is commonly referred to as the "fill factor" of the array. The fill factor, which is thus a measure of the softening of the knee, is an indication that there is some increased series resistance taking place in the array.
Other types of degradation of an array cause other particular changes in its I-V characteristic. Consequently, it is desirable to measure the I-V characteristic of an array under operating conditions in the field of determine the nature and extent of its degradation in order that proper steps can be taken to maintain the array operating at, or very near, its design I-V characteristic.
To measure the I-V characteristic of an array in the field, it is very important that the current and voltage measurements be made relative to a known solar radiation level, because obviously the array electrical output is dependent on the radiation received at the moment of the measurement. In addition to an accurate measurement of short circuit current at a known solar intensity it is also necessary to measure with accuracy the maximum power output which occurs at the center of the knee of the I-V characteristic, because many types of degradations will manifest themselves only in degradation of the maximum power point. Consequently, to test a solar cell under operating conditions it is necessary to measure the performance of the solar array under a full range of load conditions including conditions at precisely its maximum power point. There is a major problem in measuring performance of a solar array at its maximum power point.
The maximum power point of a solar array is usually measured by adjusting the output of the array using a variable power supply to buck the array output from its maximum voltage output down through the maximum power point in the knee of the I-V characteristic curve. Alternatively, it would be possible to connect a large potentiometer across the array to plot the I-V characteristic curve from its maximum voltage output at open circuit through the knee of the curve to the short circuit current measurement, but since the I-V characteristics are different for different sizes and arrangements of the arrays, it would require a different size potentiometer for the different arrays in order to dissipate the different amounts of power of the arrays while making the I-V characteristic measurements. To avoid having so many different potentiometers, it is more common practice to use a bucking power supply, but it is only feasible to do that in a laboratory, and not in the field, becase it is not feasible to attempt to build a portable instrument with some kind of large power supply (typically 500 to 1000 watts) to match the power out of the solar array if the portable instrument is itself to be powered by storage batteries.
The need for a light portable instrument to make I-V characteristic measurements of an array and to plot the actual I-V characteristic curve itself for array degradation analysis in the field, has become a problem because solar array installations are being made in locations accessible only on foot, or from a hovering helicopter. To test solar cells in those locations, the technician must have a very portable VIP meter. This need for a very portable meter will increase as solar arrays come into more widespread use because, as noted hereinbefore, they are most cost effective in the worst possible places to get to, namely places where unattended power supplies are required such as on oil platforms in the middle of a swamp, towers on mountain peaks, or towers in the middle of a desert.